The present invention relates to seating units having a comfort surface coupled to a framework and constructed to provide comfortable support to a seated user while allowing a reduction in beam strength of the framework. However, the present invention is contemplated to be substantially broader in scope than seating.
Some modern chairs incorporate tensioned fabrics to support a seated user, because tensioned fabrics provide a distinctive appearance, and potentially allow air flow to the seated user for increased comfort. However, a problem with tensioned fabrics is that the tension in the fabric must be great enough to avoid a “hammock-like” feel where the user sinks into and becomes “trapped” within (and experiences side pressure from) the fabric material. While this hammock-like feel may be acceptable for relaxing outdoors, it is not conducive or comfortable in a task chair while trying to do work. The tension required to prevent this “hammock-like” feel is considerable, and accordingly it takes a very strong frame to provide an acceptable amount of strength to adequately tension the fabric. Further, the process of pretensioning the fabric in the frame is a more difficult manufacturing step. Also, the frame strength required to support fabric under “high” tension requires mass, strong/heavy/specialized materials, and large cross-sectional sizes, all of which are undesirable in sleek-looking chair designs. However, mass and high-strength specialized materials add to the weight and cost of a product, which is highly undesirable in the competitive furniture industry.
One of the reasons that the frame must be “very strong” is because of engineering dynamics that occur on the perimeter frame members when using tensioned fabrics. When pulled tight, the fabric defines a line between the opposing edges of the fabric (i.e. a line between the side frame members supporting the opposing edges of the fabric). By pressing at a middle point between the opposing edges, a small force on the middle point generates very large inward forces on the opposing edges of the fabric. Thus, when a person sits in the chair, the initial inwardly-directed forces on the opposing perimeter frame sections are very large. The chair frame must be strong enough to resist such large inward forces, both at the instant in time when they are present, and also over time to prevent creep and permanent deformation that occurs over time (and which results in loss of fabric tension). Second, the direction of forces that the opposing perimeter frame sections must generate changes when a person sits in the chair as compared to when the chair is unoccupied. Specifically, when no-one is seated in the chair, the forces define a line parallel the sheet. When a person is seated, the vector forces change to a new direction that is a combination of the seated user's downward weight and the horizontal forces generated to maintain tension in the fabric. In order to adequately withstand the changing vectoral forces (i.e. to withstand the forces and changing directions of those forces), the perimeter frame members must provide sufficient strength and bending strength in all required directions. Hence, the problem of cross-sectional size and beam strength in a given perimeter frame member is not limited to a single direction.
Thus, a system having the aforementioned advantages and solving the aforementioned problems is desired.